This invention relates to capacitive pressure and temperature sensing devices for use at cryogenic temperatures, and in particular, to the use of quantum ferroelectric ceramic materials in such devices.
Conventional cryogenic devices such as resistance thermometers have several characteristics that degrade their accuracy, make them difficult to use, or both. More recently, work has been done with a class of materials which exhibit the property of quantum ferroelectricity. A typical ferroelectric material has a transition temperature, T.sub.C, above which the material is paraelectric and below which it is in an ordered ferroelectric state. If the transition temperature is lowered to near zero degrees (0 K.) Kelvin, then it is possible for quantum fluctuations to suppress the polar ordering. The result is a dielectric constant which is almost temperature independent at low temperatures.
Recent studies have concentrated on crystals of KTaO.sub.3 and SrTiO.sub.3 and associated solid solutions of these compounds which exhibit quantum ferroelectric behavior at temperatures near 0 K. It is known that the transition temperature in these crystals can be changed by substitution of small amounts of different ions in the compound. For example, niobium can be substituted for tantalum in KTaO.sub.3.
While these materials have been demonstrated to be suitable for use as cryogenic thermometers, see, for example, Lawless U.S. Pat. No. 3,649,891, neither of these compounds would be suitable for a pressure sensor. Ceramics in the KTaO.sub.3 system have uncontrollable losses of K.sub.2 O during ceramic formation, and single crystals of the material are prohibitively expensive. The SrTiO.sub.3 ceramics suffer from two disadvantages. These are that required refractory ceraming temperatures in the range of approximately 1500.degree. C. lead to irreproducibilities and loss of dopant or substitutional ions and, more seriously, the quantum ferroelectric behavior appears limited to the temperature range of less than 4 K.
Accordingly, there is a need in the art for capacitive pressure and temperature sensing devices and materials which are operable over a wider range of temperatures near 0 K.